In proton exchange membrane (PEM) fuel cells, hydrogen (H2) is the anode reactant (i.e. fuel) and oxygen is the cathode reactant (i.e. oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst fouling constituents, such as carbon monoxide (CO).
For vehicular applications, it is desirable to use a liquid fuel such as alcohols (e.g. methanol or ethanol), other hydrocarbons (e.g. gasoline), and/or mixtures thereof (e.g. blends of ethanol/methanol and gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard, and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam (and sometimes air) to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide is also produced requiring additional reaction processes. In a gasoline reformation process, steam, air and gasoline are reacted in a primary reactor which performs two reactions. One is a partial oxidation reaction, where air reacts with the fuel exothermally, and the other is a steam reforming reaction, where steam reacts with the fuel endothermically. The primary reactor produces hydrogen, carbon dioxide, carbon monoxide and water.
Reactors downstream of the primary reactor are required to lower the CO concentration in the hydrogen-rich reformate to levels tolerable in the fuel cell stack. Downstream reactors may include a water/gas shift (WGS) reactor and a preferential oxidizer (PROX) reactor. The PROX selectively oxidizes carbon monoxide in the presence of hydrogen to produce carbon dioxide (CO2), using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2. Unfortunately, the preferential oxidation reactor is not 100% selective and results in consumption of hydrogen. The heat generated from the preferential oxidation reactor is at a low temperature, resulting in excess low-grade heat.
The operational gasoline fuel processor technologies to date do not meet automotive targets for start-up durations, mass, and volume. The start-up time for such a system is limited by the time delay until the combination of water gas shift and preferential oxidation reactors can supply stack grade hydrogen. The start-up duration is related to the mass of the catalyst system used for start-up and the energy needed to get the catalyst system up to its operating temperature. Another limitation of the current technology is the inability to utilize the low grade heat such a system generates. Any heat loss reduces the fuel processor thermal efficiency.
Thus, it is an object of the present invention to provide a fuel processor for a hydrogen fuel cell engine which provides a means to reduce the carbon monoxide content under normal operation before entering the fuel cell stack, thereby advantageously eliminating the use of a preferential oxidizer (PROX) reactor. It is a further object of the present invention to provide such a fuel processor which provides quick carbon monoxide uptake during start-up, thereby advantageously shortening start-up duration.